MEMS microphones, also known as acoustic transducers, have been in research and development for many years. MEMS microphones have been widely used in many applications, such as cell phones, tablet PCs, cameras, hearing aids, smart toys, surveillance devices, and the like.
U.S. Pat. No. 6,781,231 discloses a MEMS package, comprising surface mountable components (e.g. silicon condenser microphones and integrated circuits), a substrate, and a cover with inner cups and outer cups, the cover being attached to the substrate to form a housing, apertures or acoustic ports formed in the cover for receiving an acoustic signal. An aperture or acoustic port may be regarded as a free “sound port” path for allowing acoustic energy to enter the inside of the housing. Each acoustic port may contain an environmental barrier layer disposed between the inner cup and the outer cup in order to prevent water, particles, and/or light from entering the package and damaging the internal components inside. However, the environmental barrier layer hinders air flow to the inside of the housing through the sound port, reducing the performance of acoustic signals to reach the micro-electromechanical system microphone.
U.S. Pat. No. 6,324,907 B1 discloses a flexible substrate transducer assembly. The flexible substrate provides connectivity between the transducer system and electronic equipment which houses the transducer assembly. A number of through-holes are formed in the second end portion of the flexible substrate to create a first passage to the external environment. An unexpected problem is that the diaphragm of the acoustic transducer in the transducer system is easily damaged due to air pressure pulses caused in drop tests.
International Patent Publication Number WO/2013097135 also discloses a MEMS microphone comprising a silicon substrate and an acoustic sensing part on the silicon substrate. A mesh-structured back hole, having a plurality of mesh beams and a plurality mesh holes defined by the mesh beams and the side wall, is formed in the substrate and aligned with the acoustic sensing part. The mesh-structured back-hole helps streamline air pressure pulses, and thus reduces the impact on the acoustic sensing part; and it can also act as a protective filter to protect alien substances such as particles from entering the microphone.
The drawback of the above two approaches is, however, that alien substances like particles are easily trapped into the diaphragm of MEMS microphone through the sound port such as holes of the flexible substrate and mesh holes of the mesh-structured back hole, especially under high air pressure pulses resulted from drop tests.